Section 6. Brain, Spinal Cord, and Nerve Disorders

Chapter 96. Cranial Nerve Disorders

Topics: Introduction | Internuclear Ophthalmoplegia | Palsies of Cranial Nerves That Control Eye Movement | Trigeminal Neuralgia | Bell's Palsy | Hemifacial Spasm | Glossopharyngeal Neuralgia | Hypoglossal Nerve Disorders

Palsies of Cranial Nerves That Control Eye Movement

These disorders involve paralysis of cranial nerve III, IV, or VI, impairing the ability to move the eyes in certain directions depending on which nerve is affected.

The eye is moved by three pairs of muscles, controlled by cranial nerves III, IV, and VI. These muscles move the eye up and down, right and left, and diagonally.

Cranial Nerve III (Oculomotor Nerve): A palsy of this nerve can be caused by brain disorders (such as a head injury, an aneurysm in an artery supplying the brain, and a brain tumor) or by diabetes. The affected eye turns outward when the unaffected eye looks straight ahead, producing double vision. The affected eye can move only to the middle when looking inward and cannot look upward and downward. The eyelid droops, and the pupil may be dilated and sometimes fixed (that is, it does not change in size). Dilation and fixation of both pupils indicates deep coma and possibly brain death (see Section 6, Chapter 84). Development of a headache and a change in the level of consciousness (for example, if the person becomes drowsy) may indicate that the cause is a life-threatening disorder.

The diagnosis is based on results of a neurologic examination and computed tomography (CT) or magnetic resonance imaging (MRI). A spinal tap (lumbar puncture (see Section 6, Chapter 77)) is performed only if doctors suspect a hemorrhage and CT does not detect blood. Cerebral angiography is performed when a hemorrhage due to an aneurysm is suspected or when the pupil is affected but no head injury has occurred. Treatment depends on the cause of the palsy. Emergency treatment is required if a life-threatening disorder is the cause.

Cranial Nerve IV (Trochlear Nerve): In most cases, the cause of a palsy of this nerve is a head injury. Other causes, such as a tumor, are rare. The affected eye cannot turn inward and downward, resulting in vertical double vision. The person tends subconsciously to tilt the head, thereby using eye muscles that are unaffected by the palsy. This position can eliminate the double vision.

Usually, the diagnosis is suspected in a person who, after a head injury, has characteristic abnormal eye movements. CT or MRI may be performed. Treatment depends on the cause of the palsy. Eye exercises may help. Sometimes surgery is necessary to eliminate double vision.

Cranial Nerve VI (Abducens Nerve): The cause may be a head injury, a tumor, diabetes, multiple sclerosis, meningitis, blockage of an artery supplying the nerve, or increased pressure within the skull. The affected eye cannot fully turn outward and may be turned inward when the person looks straight ahead. Double vision results when the person looks toward the side of the affected eye.

Usually, doctors can easily identify a palsy of cranial nerve VI, but the cause is less obvious. CT or MRI is performed to exclude tumors. A spinal tap (lumbar puncture) can determine whether pressure within the skull is increased and whether a tumor or swelling due to an infection is compressing the nerve. When no cause is identified, the cause is often thought to be blockage of an artery supplying the nerve or a transient ischemic attack that affects the nerve. These disorders are commonly the cause in people who have high blood pressure, diabetes, or atherosclerosis.

Treatment depends on the cause of the palsy. When the cause is treated, the palsy usually resolves. If the cause is a blockage of a blood vessel, the nerve regenerates, and the palsy usually resolves without treatment within 2 months.

See the sidebar Horner's Syndrome: A Droopy Eye.